Engineering with bound states in the continuum

The BIC—a beautiful phenomenon that occurs in the many fields of wave physics, including electromagnetism, acoustics, hydrodynamics and quantum mechanics—is finding a new life in optics as well. And, as even the work that has been done thus far has shown, that development could have important implications for engineering highly efficient resonances for photonics at the nanoscale. One very recent demonstration, for example, goes beyond the traditional concept of the interference of leaky modes to create BICs, employing the interference of multiple BICs themselves in an optical resonator—thereby substantially suppressing optical scattering losses due to roughness and structural disorder of the material itself, by merging BICs with different propagation directions. This enables dramatic enhancement of the optical Q factor to experimentally record-high values of 4.9×105—more than an order of magnitude higher than any earlier reported BIC observations. The possibility of converting optical waves into BICs also allows the realization of supercavity modes characterized by extremely high Q factors in resonators with very compact geometries. Many high-refractive-index dielectric materials involving the vectorial nature of electromagnetic waves could be employed to reduce the resonator dimensions, and to combine individual, high-Q BIC resonators in structured arrays. We envision the rapid development of novel approaches in the electromagnetic theory of vectorial resonances in such arrays in the future. By engineering Qfactors in the BIC regime in this way, we believe that future technologists will be able to substantially enhance nonlinear and quantum effects—and thus to develop low-threshold lasers and realize strong-coupling regimes of nanoscale supercavities. © 2020 OSA - The Optical Society. All rights reserved.